1. Field of the Invention
This invention relates to polymer-based filaments and fibers and methods of reinforcement using nanotubes.
2. Description of the Related Art
Nearly all carbon fiber composite materials are based on carbon fibers which are bundles of thousands of carbon filaments of 5-10 microns in diameter. The filaments are composed of stacked graphene sheets held together by Van der Waals forces produced from precursors and a multistep manufacturing process. Typical carbon fiber precursors are polyacrylonitrile (PAN), mesophase pitch, or rayon. The mechanical characteristics of the fiber depend on the precursor material, optimization of processes to form fibers, and processing temperatures.
The process for making carbon fibers is part chemical and part mechanical. The precursor is drawn into long strands or fibers and then heated to a very high temperature without allowing it to come in contact with oxygen. Without oxygen, the fiber cannot burn. Instead, the high temperature causes the atoms in the fiber to vibrate violently until most of the non-carbon atoms are expelled. This process is called carbonization and leaves a fiber composed of long, tightly inter-locked chains of carbon atoms with only a few non-carbon atoms remaining.
A typical sequence of spinning, stabilizing, carbonizing, surface treatment and sizing to produce a carbon fiber from a PAN precursor is illustrated in FIG. 1. A monomer having a carbon-carbon backbone such as acrylonitrile (CH2CHCN) is mixed (step 10) with a catalyst. The mixture 12 is polymerized (step 14) to form a PAN polymer 16 including long polymer chains formed from the monomers. The PAN polymer is spun (step 18) into filaments 20, washed and stretched (step 22) to produce aligned filaments 24 in which the polymer chains are aligned along a common axis and having a final diameter. After spinning, the filaments are heated in air (200-300 degrees C for up to 120 minutes) (step 26) to form poly/poly bonds between individual polymeric chains to produce stabilized filaments 28. The stabilized filaments are heated (carbonized) (step 30) in an atmosphere without oxygen at temperatures of 1000-3000 degrees C. to produce carbonized filaments 32. This removes non-carbon atoms from the filaments and forms more tightly bonded carbon atoms parallel to the long axis of the filament. This process promotes the formation of graphite flakes in the filaments. After carbonizing, the surface of the filaments are oxidized (step 34) to form a coated filament 36 to provide better chemical bonding properties and to roughen the surface for better mechanical bonding properties. The coated filaments are coated (sized) with materials compatible with the adhesives used to form composite materials and spun (step 38) into the macro carbon fiber 40.
Carbon fibers have the highest tensile strength of reinforcements for composite materials, but improvements in the mechanical properties can be made by adding other materials during their synthesis. Adding carbon nanotubes (CNT) to carbon fibers also allows better control of the thermal and electrical properties of the final material. Researchers have introduced single and multi-walled CNTs which have superior mechanical properties to carbon fibers to the synthesis process of carbon fibers (See Thaliyil V. Sreekumar et al. “Polyacrylonitrile Single-Walled Carbon Nanotube Composite Fibers” Adv. Mater. 2004, 16, No. 1, January 5, pp. 58-61). The CNTs 42 are added (step 44) after the acrylonitrile has been polymerized in step 14 to form PAN polymer 16. As shown in FIG. 2, poly/poly bonds 46 are formed between PAN polymer chains 16, bonds 48 are formed between PAN polymer chains 16 and CNTs 42 only at the surface of the CNTs and carbon/carbon bonds 50 are formed between CNTs 42 and graphite flakes 52. The CNTs are encapsulated in the bonded polymer chains but the number of actual bonds to the CNTs is limited. Poor bonding of CNTs limits the increases in mechanical strength of the resulting carbon fibers.